1. Field of the Invention
The present invention generally relates to an optical communication system using an optical modulator having polarization dependency, and more specifically, to a technology of removing non-uniform amplification factors of modulating signals of the optical modulator having polarization dependency by mixing two lights whose polarization directions are vertical each other and using them as a light source.
2. Description of the Drawings
FIG. 1 is a mimetic diagram illustrating a conventional optical communication system using an optical modulator having polarization dependency. Since a reflective semiconductor optical amplifier is usefully applied in a Wavelength Division Multiplexer-Passive Optical Network (hereinafter, referred to as “WDM-PON”) as disclosed in “Semiconductor Laser Amplifier-Reflector for the Future FTTH Applications” (Vol. 2, p. 196) written by N. Buldawwo, S. Mottet, F. le Gall, D. Siggogne, D. Meichenin, S. Chelle in European Conference on Optical Communication, an example where the reflective semiconductor optical amplifier is used as an optical modulator having polarization dependency.
In general, the optical communication system comprises a light source 100, an optical circulator 200, a reflective semiconductor optical amplifier 300 and a photodiode 400.
An optical generator 110 in the light source 100 outputs a continuous wave (hereinafter, abbreviated as “CW”) laser light λs having a predetermined strength, and the representative example of the optical generator 110 is a Distributed Feedback Laser Diode (hereinafter, referred to as “DFB-LD”). The laser light λs from the optical generator 110 is projected into the reflective semiconductor optical amplifier 300 through the optical circulator. Then, the reflective semiconductor optical amplifier 300 receives the laser light λs to generate a modulated optical signal λm that has the same wavelength as that of the projected laser light λs. The optical signal λm modulated by the semiconductor optical amplifier 300 is inputted in the photodiode 400 through the optical circulator 200, and then transformed into an electric signal.
FIG. 2 is a diagram illustrating the operation principle of the reflective semiconductor optical amplifier 300 of FIG. 1. The projected light λs inputted through the optical waveguide 310 is transmitted into the active layer waveguide 320, reflected in the high reflection coating film 330, and outputted to the optical waveguide 310 again. Here, while the projected light λs is processed along the active waveguide 330, the projected light λs is amplified depending on inputted signal current. Since a reflected output light copies the signal current, the reflective semiconductor optical amplifier 300 converts the light λs into the upstream optical signal λm having the same wavelength as that of the light λs.
Referring to FIG. 3, the active layer waveguide 320 has a quantum well structure which is formed of alternately deposited materials having a large energy band gap and a small energy band gap at tens of Ω. In this case, the state available in a Momentum space is two-dimensionally distributed, and the state depending on energy levels is intensively distributed in a specific energy level to increase photoelectric conversion efficiency. However, since a quantum well layer has its horizontal structure different from its vertical structure, the distribution in the momentum direction of excited electrons is directional. A light projected into the semiconductor optical amplifier having a quantum well structure has different photoelectric conversion efficiency depending on its polarization direction. That is, in general, optical amplification gain by the semiconductor optical amplifier having a quantum well depends largely on the polarization direction of the projected light.
For example, as shown in FIG. 4, the optical amplification gain is 20 dB when the projected light is vertically polarized while the optical amplification gain is 10 dB when the projected light is horizontally polarized.
Meanwhile, the laser light λs generated from the optical generator 110 by duplicating photons has polarization because a polarization characteristic is also duplicated during the above generation process. As a result, when a single mode laser light having the polarization is used in an optical communication network, its polarization direction can be polarized due to distortion of an optical fiber. However, when the polarized laser light is inputted and then modulated in the reflective optical amplifier 300 having large polarization dependency of the optical amplification gain, the modulated optical signal has a difference of the optical amplification gain as described above, which causes instability of the optical transmission system.